Impulse electrostatic printing apparatus and method



Aug. 29, 1967 H. M. OWREN 3,338,161

IMPULSE ELECTROSTATIC PRINTING APPARATUS AND METHOD Filed June 16, 1965 2 Sheets-Sheet 1 FIG 3 4 E 29 53 x 3 I.

v INVENTOR. HARVEY M. OWREN h Mam ATTORNEYS Aug. 29, 1967 H. M. OWREN 3,338,161

IMPULSE ELECTROSTATIC PRINTING APPARATUS AND METHOD Filed June 16, 1965 2 Sheets-Sheet 2 INVENTOR. HARVEY M. OWREN 26%,, My? {*Q ATTORNEYS United States Patent Ofiiice Patented Aug. 29, 1967 3,338,161 IMPULSE ELECTROSTATIC PRINTING APPARATUS AND METHOD Harvey M. Owren, Livermore, Calif., assignor to Unrmark Corporation, San Ramon, Califl, a corporation of California Filed June 16, 1965, Ser. No. 464,311 3 Claims. (Cl. 101-129) This invention relates to electrostatic printing, and more particularly to improvements in the electrostatic printing process as disclosed in United States Letters Patent No. 3,081,698 of Mar. 19, 1963, issued to Childress et al., and in copending application Ser. No. 366,981, filed May 13, 1964.

One of the objects of this invention is the provision of improved apparatus for higher speed electrostatic printing, without the necessity of moving the stencil or screen, than heretofore.

A still further object of the invention is the elimination of brushes or brush rollers for delivering the printing powder to the open areas of the stencil as heretofore.

An added object of the invention is the provision of improved apparatus for electrostatically printing on surfaces of uneven shape or contours than heretofore.

A still further object of the invention is the provision of electrostatic printing apparatus which allows greater separation between the stencil or screen and the surface to be printed upon than heretofore.

An additional object of this invention'is the provision of an improved system in which uniform air impulses of high intensity are appliedto the printing medium which overlies or is carried by the stencil or screen while articles to be printed upon are spaced below and passed beneath the stencil or screen at a rate of speed such that the object to be rinted upon is beneath the screen when the impulse is applied to the printing medium.

A still further object of the invention is the provision of apparatus, and a method or process, for eliminating the disadvantages attached to the electrostatic printing method that heretofore has required brushing the surface dry printing powders through the screen or stencil. Where such powders are abrasive the screen and brush have a limited life and become progressively less efiicient. By the present invention no brush is employed, and the screen has a long and highly eflicient life.

An added object is the elimination of distortion of the screen that heretofore has occurred where brushes are used to force the powder through the screen, and furthermore, the present invention enables the use of a screen having a compound curve.

Coarser powders, not adapted for brushing may also be employed, and the added velocity due to air impulses instead of the use of brushes contributes to the higher printing speed than heretofore.

In explanation, there is provided a fine-mesh printing screen or stencil consisting, as one example, of over three hundred mesh openings per inch. Certain openings in the screen are sealed, or marked 03, with a coating while the screen openings which make up the print pattern are left open. A printing powder, capable of passing through the open mesh interstices which make up the print pattern on the stencil, is placed on the screen. Beneath the screen is a carrier for carrying individual articles such as transistors, apples, eggs, potatoes, avocados, etc. A potential of a predetermined number of volts is applied between the article to be printed upon and the aforesaid screen. As each article passes below the screen, a substantially uniform air impulse is applied to the powder, forcing it through the openings in the screen, to the opposite side thereof and from said opposite side across an air gap by reason of the electrostatic field, and onto the intended article. The powder will impinge on the article in the form of a layer having an outline corresponding to the pattern or open areas on the screen. This sequence of events is repeated as each object to be printed upon passes beneath the printing screen.

The application of a uniform air impulse to the printing powder overlying the printing screen or to powder eifectively supported within the influence of such an impulse for movement toward the article to be printed therewith, and the application of a potential of from say several thousand volts up, according to the circumstances, between the screen and the object to be printed upon, provides a marked improvement in the field of electrostatic printing.

Other objects and advantages will be obvious from the following description when read in connection with the accompanying drawings.

In the drawings:

FIG. 1 is a semidiagrammatic, simplified view of one form of the invention.

FIG. 2 is a semidiagrammatic, simplified view of a modification of the invention shown in FIG. 1.

FIG. 3 is a semischematic view of apparatus incorporating the invention shown in FIG. 1, in which the articles to be. printed upon are carried past a printing station, and which articles may be units having generally convex outersurfaces.

FIG. 4 is a semischematic view of apparatus similar to that of FIG. 3, but incorporating the invention as shown in FIG. 2.

FIG. 5 is an enlarged fragmentary perspective view showing the printing screen and an article, such as shown in FIGS. 1 and 2 in printing relation to the screen after the image has been transferred to the article.

FIG. 6*is a fragmentary view showing apparatus incorporating the invention as shown in FIG. 1 but in rela tion to a sheet of material to be printed upon.

FIG. 7 is a fragmentary view showing apparatus incorporating the invention as shown in FIG. 1 in relation to a rigid sheet of material that is to be printed upon.

In FIG. 1 an impulse generator, generally designated form through the area below the diaphragm.

Below the diaphragm, and spaced therefrom, is the stencil or screen '8, which may be of any suitable material. A woven fine mesh screen of nylon has been found to be suiable, and the design of open areas 9 of the screen, around which areas the mesh openings are stopped out, as at 10 (FIG. 5). One method of preparing the screen -8 is disclosed in the aforesaid US. Patent 3,081,698. A suitable electrically conductive plate or frame 13 may support said screen.

Spaced below screen 8 is the article 15 (FIG. 1) onto which the design 9 is to be imprinted. In FIG. 5 the article 16 to receive the design is shown with a convex surface and with the design deposited thereon. Otherwise the element in FIG. 1 would be the same. A conductor 17 electrically connects the positive terminal of a D.C. power source 18, which may be say 3,000 volts as an example, with the article 15 (or 16) and the negative terminal of said power source is connected by conductor 19 with the frame 13 of screen 8.

The printing powder to be used in the present invention is in the form of fine, surface-dry particles having a particle size sufficiently small to pass through the stencil openings, or mesh openings, in the screen 8, and a thin layer of such powder covers at least the open areas 9 of the screen, although it normally covers substantially the entire upper surface of the latter. These particles that are on the screen do not, by gravity, pass through the mesh openings due to the normal adhesion of the particles.

When the article 15 (or 16) is positioned directly below the print pattern 9 of the printing screen 8, and relatively close to the latter a distance of say approximately ,5 of an inch, or at a distance suflicient to prevent arcing between the positively charged article 15 and the negatively charged screen :8, and the switch is closed, the transducer is :acuated, whereby a number of rapid air impulses or waves are generated by the movement of diaphragm 6, which air impulses impart sufficient energy to the screen to cause the powder particles to move through the openings in the screen and into the electrostatic field between the screen and the positively charged article to be quickly deposited on the article by any suitable fixative such as vapor or spray from an atomizer or spray nozzle generally designated 12, such as disclosed in copending US. application Ser. No. 366,981, filed May 13, 1964, by Martin A. Schueler.

Preferably, by using or generating a number of pulses a uniform print on the article is made, and increasing the energy per pulse, would proportionately decrease the number of pulses required.

The size of the transducer diaphragm, and the voltages used and the number of impulses generated may obviously vary with the size of the imprint to be made, and the spaces between the diaphragm, screen and article 15 and other factors.

Referring to FIG. 2 the shock wave or air impulse is produced in a different manner. In this view the impulse generator is generally designated 20 and comprises a pair of electrodes 23, such as tungsten or carbon tipped rods, Whose tips are spaced apart to provide a small air gap between them. A 10,000 volt DC. power supply 24, as an example, is connected with one of the rods 23 by a conductor 25 having a switch 26 therein, and the other rod 23 is connected with power supply 24 by a conductor 27. A capacitor 28, of 1 to microfarads, is electrically conuected in parallel with the carbon rods 23.

In FIG. 2 the electrodes 23 are horizontally aligned, and spaced completely around said electrodes is a stationary, resilient, thin metal, container 29, the spark gap between said electrodes 23 being substantially centrally disposed within said container 29. Extending outwardly from one side of the container 29 is a tube 31 which is vented to the atmosphere.

Spaced below the container 29 is the printing screen 30 that may be identical with screen 8. Screen 30 is horizontal and is stationarily supported in a conductive frame 33, and spaced below screen 30 is the article 34 to be printed upon. The screens 8 and 30, as well as frames 13 and 33 may be identical.

A conductor 35 connects the positive terminal of a D.C. power source 36 with the article 34 and the negative terminal of said power source is connected by a conductor 37 with the frame 33 of screen 30.

When the screen 30 is covered with the printing powder in the same manner as described with respect to the screen 8, and switch 26 is momentarily closed, a spark is produced at the gap between the electrodes 23 and an air blast or shock wave is produced. This wave strikes the inner walls of the container 29, and while much of the wave is dissipated via tube 31, nevertheless the bottom wall 32 of said container 29 is actuated to produce a pressure wave between said bottom wall 32 and the screen 30 and the powder on the latter to impart suflicient energy to the screen to force the powder on the screen through the open mesh openings in the latter and into the electrostatic field between the screen '30 and the article 34 for being carried onto the surface of the positively charged article 34 for later fixing thereon. Thus the impulse generator 20 may properly be called a transducer.

When the container 29 is omitted, the shock wave generated by the spark will directly strike the screen and powder to accelerate the screen and powder differently and thereby cause the powder to be forced through the open mesh openings in the screen; a more accurate image is produced on articles 34 under all conditions when container 29 is used than when it is not. It was found that when the energy discharged in the spark gap was increased beyond a point, it would cause some scattering of powder on the print, which was apparently caused by the air blast from the late part of the are, instead of the initial shock. The use of the container 29 eliminated this scattering, and its use also enabled accurate printing with less concern over the amount of powder on the screen. Fur thermore, the use of a container isolates the spark from the powder and reduces the hazard of a powder explosion.

Referring to FIG. 3 the air impulse generator 1 dis-' closed in FIG. 1 is incorporated into a carrier system, and the same numbers used in FIG. 1 will be used for identifying elements in FIG. 3 that correspond to those used in FIG. 1. It is to be noted that the articles 16 are shown in FIG. 3 instead of the planar articles 15 shown in FIG. 1.

In FIG. 3 the electromagnetic transducer 2, diaphragm 6 thereof, screen 8 and article 16 are in the same relation to each other as disclosed in FIGS. 1 and 5, and the impulse generator as a whole is stationarily supported above a horizontally disposed article supporting conveyor generally designated 38.

Convey-or 38 comprises a pair of corresponding endless sprocket chains 39 that extend at one of their ends over corresponding sprocket wheels 40 secured on a shaft 41 that is connected with a motor 42 for moving the upper parallel runs of chains 39 from right to left as seen in FIG. 3. A pair of corresponding sprocket wheels 43 on a shaft 44 support the chains 39 at their ends remote from sprocket wheels 40.

Equally spaced article supports 46 are secured on links of the chains 39, which supports are of electrically conductive material and have convergently downwardly extending sides 47 for supporting each article 16 centered in each support 46.

The upper runs of chains 39 carry the supports 46 so the article 16 in each support will be carried directly below the screen 8 the desired distance from the screen, and rigid strips 21 below the portions of the chains 39 below the impulse generator 1 support the chains against sagging to insure the supports 46 being at a uniform height as the supports 46 move below and past the impulse generator.

An electrical insulator 22 between the chains 39 and each article holder or support 46 insulates each such holder or support from the chains and an electrical conductor member 48 on each holder projects therefrom for sliding engagement with a yieldable stationary conductor 49 that, in turn, is connected with the conductor 17 leading from the DC. supply source 18. Upon the article 16 being at the point centered below the impulse generator, it will be seen that the electrical potential between the screen 8 and article 16 will be established to provide for conducting the powder from the screen 8 onto the article. The holder 46 and conductors 48, 49 provide continuations of the conductor 17 to establish the potential between article 16 and screen 8.

Connected to each insulator 47 is an arm 50, and in the path of each such arm is switch actuating arm 51 of a microswitch 52 instead of the switch 5 of FIG. 1. This microswitch is normally open and automatically returns to open position when not held in closed position by the actuating arm 51.

The conductors 3 leading from the power source 4 extend to the switch 52.

Said switch actuating arm is supported in a position to be engaged and actuated for any desired duration by movement of arm 50 when an article 16 is substantially centered below the impulse generator so as to give the desired impulses. Normally this is a period sufliciently short to detect no noticeable smear of the powder on the article when the latter is moving at fifty feet per minute and more.

A typical distance between the screen and the diaphragm 6 of FIGS. 1, 3 or between the screen and the diaphragm of any of the forms of invention employing the'impulse generator of FIG. 1 is two inches and the same is true of the distance between any forms of the invention in which the transducer of FIG. 2 is employed.

In operation it appears that any transducer with a time constant of approximately .001 second is suitable for pulsing the powder, and the use of a number of pulses during each printing operation is usually preferable. The number of pulses so used may be reduced by increasing the energy per pulse, and the imprints on the articles are satisfactory where the print receiving surface is flat, convex, or irregular.

The impulse generator, screen 8, conductor 49 and microswitch 52 are all supported in the relationship heretofore described on any suitable frame, generally designated 45, and which frame may also support shafts 41, 43 and the other elements of the combination shown in FIG. 1 as well as the similar elements in FIGS. 4-7, and said frame also supports powder distribution means for depositing a layer of powder on screen 8 and a powder fixing means generally designated 12 is provided at a point over the conveyor beyond the printing station for fixing the powder on the article on which it has been deposited, by any suitable means such as solvent vapor, spray, or by heat, generally as disclosed in the patent and application for patent mentioned hereinbefore.

Referring to FIG. 4, the transducer arrangement of FIG. 2 is employed and the carrier system of FIG. 3 is used, hence the same numbers are used on the elements of the carrier system of FIG. 1 and the transducer of FIG. 2.

In'both FIGS. 3 and 4 any suitable means generally designated 53 may be employed to deposit the printing powder onto the screen, between actuations of the respective transducers, such as a mechanical or air actuated discharge nozzle connected with a source of powder, or in FIGS. 1, 2, the powder may be manually applied.

- Only a portion of the conveyor system of FIG. 3 is shown in FIG. 4 since the same system is applicable to both views. Upon the holders 46 carrying the articles 16 reaching substantially a point centered directly below the screen 30, the microswitch 52 will be actuated momentarily, as described in FIG. 3 to cause a spark between the rods 23 and the energy to effect the relative movement between the screen 30 and the powder thereover for causing the powder to pass through the mesh openings in the screen and into the electrostatic field therebelow caused by the closing of the switch elements 48, 49 in between conductor 35 and holder 46.

It is to be understood that the period when the circuit from the D.C. power supply is closed may be synchronized to exist only substantially during the period when the circult from the driving source 24 is closed, since the transfer of the powder from the screen 30 onto the article 16 or is a sutficiently small fraction of a second to prevent noticeable smear in the print during movement of the article 16 past the screen 8.

FIG. 6 is merely a variation in the carrier or conveyor system of FIG. 3, but either the transducer of FIGS. 1 or 2 may be used. The transducer illustrated is the one shown in FIG. 1.

In FIG. 6 a long, flat article 54' of any material of any desired "length may be supported on an endless flat conveyor belt 55 having spaced sections 56 of electrical conductive material separated by sections 57 of electrical insulation material. Sections 56 of electrical conductive material are successively moved 'below and past screen 8. An electrical conductive arm 58 secured to each conductive section 'or area 56 engages the yieldable conductor member 49 connected with the conductor 17, which latter conductor is connected with the positive terminal of the D.C. supply source 18. The arm 58 engages the yieldable conductor 49 in exactly the same manner as arm. 48 on each holder 47 in FIG. 3 engages the yieldable conduct-or 49.

Secured to each insulated portion 57 of conveyor 55 is an arm; 59 that corresponds to each arm 50in FIG. 3, and which arm actuates the microswitch 52 in the same manner as the microswitch 52 of FIG. 3 is actuated, the latter being in the circuit from the D.C. power supply.

Belt 55 carries the material 54' from right to left as seen in FIG. 6, and the portion of material 54' that is below the transducer will receive the deposit of powder from the screen immediately upon closing of the power circuits to the power supplies 4 and 18 (not shown in FIG. 6) which are the same as in FIG. 3, and which circuits are closed in exactly the same manner as in FIG. 1.

In FIG. 7 the transducer is the same as in FIG. 1 and 3, but the article to be printed upon in this instance [may he a strip of relatively rigid material such as plywood. Such strip 60 may the driven past the transducer 'by pairs of power driven rollers 61, 62 on shafts 63, 64. The latter have inter-meshing power driven gears 65, 66 secured thereon respectively connected with any suitable source of power for driving the rollers, such as a motor 67.

A roller 68 below the transducer and below strip 60 has electrical conductive sections 69 around its outer periphery spaced by sections '70 of electrical insulation materiai. The same electrical system is used in as in FIG. 1, except that the positive terminal of the D.C. supply is connected with a wiper arm 73 that, in turn, successively engages conductive sections 74 of an armature which sections are respectively electrically connected with the sections 69.

A shaft 75 carries roller 68 and is electrically insulated from the armature sections 74. This roller 68 may be driven by. the material 60 itself or 'by motor 67, and projecting from one of the ends of the roller, and electrically insulated from the sections 69 and 74 are projections 77 that are respectively adapted to actuate the microswitch 78 that corresponds to the microswitch 52 in the electrical circuit of FIG. 3 upon each section 69 being in engagement with the strip 60 for etfecting actuation of the trans ducer 2, wherebythe powder on screen 8 will be moved gfgough the open mesh of the screen 8 onto the material In each of the examples shown herein, the voltage of the D.C. power source, such as designated 18 in FIGS. 1, 3, may obviously vary according to the distance between the screen and the article or material that is to be imprinted with powder in a design corresponding, to that defined by the open mesh openings in the screen.

The articles, such as indicated at 16 in FIGS. 3, 4 and 5, are shown as spherical, for simplicity, but these may be of irregular contour such as potatoes, eggs, avocados, etc., and the surface contour of articles such as designated 15, 34, 54 and 60 may be corrugated, such as corrugated cardboard or metal sheets, or articles havng a rough and irregular surface that cannot receive printmg by direct engagement with a printing roller or plate without injury to the surface. In the present instance the printing will be accurate and unbroken on the projections and on the surfaces of cavities or recesses between the proections.

From the foregoing explanation, it is seen that all of the disadvantages of employing brushes or mechanical means to cause the powder to pass through the openings in the screen are overcome. Also,- by the present invention the use of a stationary stencil screen for printing on a moving article, without noticeable smear, is accomplished,

7 thereby eliminating the problem incurred in having perfect synchronization between the movement of the screen with the article, and at the same time accelerating the printiug time.

The inclusion in this description of exact voltages and breaks the circuit, and after the circuit is broken, in FIG.

2, the capacitor 28 is recharged by power supply 24 to its prior voltage level. r

The fixing means for fixing the powder deposited on the article or material to receive the image may, as has been described, be by spraying the deposited powder with a solvent in spray or vapor form, or it may be by application of heat :or any other suitable means according to the characteristic of the powder. The present invention is not concerned with any specific means for fixing the powder, nor the specific powder supply means 53, since the latter may be any suitable powder discharge or feed device that is adapted to discharge powder in a relatively thin layer onto the screen 8.

Insofar as FIGS. 1 and 2 are concerned, there is no relative movement between the transducer, screen 8 and article 15 at the time the transducer is actuated, and the same is true of the transducer, container 29, screen 30 and article 34 of FIG. 2, consequently the problem of powder smear conducted onto the articles 15, 34 due to relative movement between the article and the elements of the impulse generators, is not present. Where there is relative movement between the impulse generator and the article the distances between the elements of the generator and between the screens and the articles to be printed upon are reduced to the point where the spaces are relatively small.

In practice, any transducer with a time constant of approximately 1 10- would be suitable for pulsing the powder, and normally a number of such pulses is preferable, but which number could no doubt be reduced by specially designed transducers *for the specific purpose of this invention.

In operation, even where the relationship between elements of the impulse generator and the article printed upon were such that the deposition time was approximate- 1y 3 X10 seconds there was no noticeable powder smear with a velocity of the article of over fifty feet per minute, althougha deposition time of approximately 1 10- seconds, or as short as possible, is preferable.

It is to be understood that the invention is not to be considered as limited by the detailed description of the embodiments herein disclosed, since changes can be made therein which are within the intended scope of the invention as defined by the appended claims.

I- claim:

1. In the method of electrostatically depositing surface dry powder supported on one side of a stencil screen and on an area thereof having openings mechanicaly 'unobstructed to passage of said powder therethrough, onto an article at the opposite side of said screen that is in spaced opposed relation to said area, comprising the steps of:

(a) moving said powder through the openings of said area to the opposite side of said screen that faces said article by applying an air impulse to the powder on said one side generally in a direction toward said opposite side, said air impulse being initiated by effecting a rapid vibration of a surface spaced from and opposed to said area in a direction substantially normal to the plane of said screen whereby the said impulse will be transmitted to said one side of said 8' screen substantially simultaneously and uniformly over the unobstructed area of said screen, and

(b) applying an electrical potential between said screen and said article suflicient to establish an electrostatic field between them for conducting said powder so moved to said opposite side of said screen onto said article.

2. In the cethod of electrostatically depositing surfacedry powder supported on one side of a stencil screen and on an area thereof having mechanically unobstructed openings :for passage of said powder therethrough and onto an article at the opposite side of said screen in spaced op posed relation to said area, comprising the steps of:

(a) moving said powder through the openings of said area to the side of said screen facing said article by generating an air impulse at a point spaced firom the side of said screen opposite to said article and applying said air impulse to the powder on said screen,

(b) dampening said air impulse in a plane spaced between said point and said screen and at the same time transmitting the energy of said impulse in said plane to air waves directed from said plane in a direction generally normal thereto to said screen and said areas, and

(c) applying an electrical potential between said screen and said article sufficient to establish an electrostatic field between them for conducting said powder so moved to the side facing said article, onto said article.

3. Apparatus for electrostatic deposition printing that comprises:

(a) a screen for supporting printing powder thereon and having mesh openings through which said printing powder of particle size to pass through said openings is adapted to be moved from one side of said screen to the opposite side fior electrostatic deposition irom said opposite side onto the material to receive said powder;

(b) means for supporting said material at said opposite side in a position spaced from said mesh openings and in direct opposition thereto;

(0) means spaced cfrom said one side and out of contact with powder on said screen for operatively eifecting relative impulsive movement between said screen and the powder thereon for simultaneously moving powder particles on said screen through said openings in the latte'r to said opposite side, said means for operative'ly effecting said relative impulsive movement being an electrically actuatable transducer for generating planar air impulses and positioned for 'di'-' recting such planar air impulses perpendicularly against said one side of said screen and printing powder supported thereon;

(d) means electrically connecting said transducer with an electrical power source for actuating said transducer, and

(e) means connected with said screen and said article supporting means for applying an electric potential between said screen and said material for conducting said powder particles at said opposite side of said screen onto said material.

References Cited UNITED STATES PATENTS 1,894,530 1/1933 Bernardo 101-114 2,975,703 3/1961 Burkhardt 1011 14 3,187,669 6/1965 Greenblott et a1. 10l-114 3,211,088 10/1965 Naima'n 1()1-l14 3,238,053 3/1966 Morgan 101129 X WILLIAM B. PENN, Primary Examiner.

ROBERT E. PULFREY, Examiner.

H, P. EWELL, Assistant Examiner. 

1. IN THE METHOD ELECTROSTATICALLY DEPOSITING SURFACEDRY POWDER SUPPORTED ON ONE SIDE OF A STENCIL SCREEN AND ON AN AREA THEREOF HAVING OPENINGS MECHANICALLY UNOBSTRUCTED TO PASSAGE OF SAID POWDER THERETHROUGH, ONTO AN ARTICLE AT THE OPPOSITE SIDE OF SAID SCREEN THAT IS IN SPACED OPPOSED RELATION TO SAID AREA, COMPRISING THE STEPS OF: (A) MOVING SAID POWDER THROUGH THE OPENINGS OF SAID AREA TO THE OPPOSITE SIDE OF SAID SCREEN THAT FACES SAID ARTICLE BY APPLYING AN AIR IMPULSE TO THE POWDER ON SAID ONE SIDE GENERALLY IN A DIRCTION TOWARD SAID OPPOSITE SIDE, SAID AIR IMPULSE BEING INITIATED BY EFFECTING A RAPID VIBRATION OF A SURFACE SPACED FROM AND OPPOSED TO SAID AREA IN A DIRECTION SUBSTANTIALLY NORMAL TO THE PLANE OF SAID SCREEN WHEREBY THE SAID IMPULSE WILL BE TRANSMITTED TO SAID ONE SIDE OF SAID SCREEN SUBSTANTIALLY SIMULTANEOUSLY AND UNIFORMLY OVER THE UNOBSTRUCTED AREA OF SAID SCREEN, AND 